Correct Answer: D. Spiral artery by extravillous trophoblasts
Preeclampsia fundamentally arises from defective placentation, specifically failure of extravillous trophoblasts (EVT) to adequately invade and remodel the spiral arteries of the uterus. During normal pregnancy, EVT migrate from the anchoring villi into the decidua and myometrium, progressively replacing the endothelium and smooth muscle of spiral arteries. This transformation converts high-resistance, narrow vessels into low-resistance, dilated conduits that can accommodate the increased blood flow demands of pregnancy. When this invasion fails—due to genetic, immunological, or environmental factors—spiral arteries remain narrow and vasoresponsive. This leads to placental hypoperfusion, oxidative stress, and release of anti-angiogenic factors (sFlt-1, soluble endoglin) into maternal circulation, triggering systemic endothelial dysfunction, hypertension, proteinuria, and the clinical syndrome of preeclampsia. This pathophysiology is well-established in Indian obstetric practice and explains why preeclampsia is a disease of the placenta, not the mother. The defect is specifically in EVT invasion of spiral arteries—not arcuate or radial arteries, which are not remodeled during pregnancy.
Why the other options are wrong
A. Arcuate artery by extravillous trophoblasts — Arcuate arteries are located in the outer myometrium and are not the primary target of trophoblastic invasion. EVT invasion is directed at spiral arteries in the decidua and inner myometrium, not arcuate arteries. This option confuses the vascular anatomy of the uterus and represents a distractor based on partial knowledge of uterine blood supply. B. Spiral artery by villous trophoblasts — Villous trophoblasts form the syncytiotrophoblast layer lining the placental villi and are involved in nutrient exchange, not vascular invasion. Extravillous trophoblasts (not villous) are the invasive cells responsible for spiral artery remodeling. This is a classic NBE trap pairing the correct vessel with the wrong cell type. C. Radial artery by cytotrophoblasts — Radial arteries are branches of arcuate arteries in the myometrium and are not significantly remodeled during pregnancy. Additionally, cytotrophoblasts are the precursor cells that differentiate into EVT; the invasive phenotype is EVT, not undifferentiated cytotrophoblasts. This option mixes incorrect vessel and cell terminology.
High-Yield Facts
- Extravillous trophoblasts (EVT) are the invasive cells that remodel spiral arteries during normal pregnancy, converting them from high-resistance to low-resistance vessels.
- Spiral artery remodeling is essential for adequate placental perfusion; failure leads to placental hypoperfusion, oxidative stress, and preeclampsia.
- Defective placentation (shallow EVT invasion) is the primary pathophysiological mechanism of preeclampsia, not maternal endothelial disease.
- Anti-angiogenic factors (sFlt-1, soluble endoglin) released from hypoxic placenta trigger maternal endothelial dysfunction and systemic preeclampsia manifestations.
- Preeclampsia is a disease of the placenta, not the mother—explaining why delivery is the only definitive cure and why maternal risk factors are less predictive than placental factors.
Mnemonics
EVT-SPIRAL Extravillous Trophoblasts invade SPIRAL arteries. Remember: EVT = invasive cells; Spiral = the vessel they remodel. Villous trophoblasts stay in villi (nutrient exchange); EVT invades vessels (hemodynamics). Preeclampsia = Placental Problem Preeclampsia = Placental pathology (defective EVT invasion). Not a maternal disease—it's a placental disease that secondarily affects the mother via anti-angiogenic factors and endothelial dysfunction.
NBE Trap
NBE pairs spiral arteries (correct vessel) with villous trophoblasts (wrong cell type) in option B to trap students who know the vessel anatomy but confuse the two trophoblastic populations. The discriminator is recognizing that extravillous (invasive) trophoblasts, not villous (exchange) trophoblasts, perform vascular remodeling.
Clinical Pearl
In Indian obstetric practice, preeclampsia remains a leading cause of maternal mortality. Understanding that it stems from placental pathology—not maternal hypertension—explains why antihypertensive agents alone do not prevent progression and why early recognition of shallow placentation on Doppler (elevated uterine artery resistance index) can identify high-risk pregnancies for prophylactic aspirin and close monitoring.
_Reference: DC Dutta's Textbook of Obstetrics Ch. 18 (Hypertensive Disorders in Pregnancy); Harrison Ch. 428 (Hypertensive Disorders in Pregnancy)_